The invention concerns an improved wheel or drum brake, especially suited for fitment to vehicles of all kinds, as well as a vehicle brake system which allows some of the energy used in braking, which is normally dissipated by heat dispersal, to be salvaged and used to power either the brake system itself, or ancillary systems, or used to provide energy for important primary and secondary features.
As may be gathered from the title, the brake itself is most suited to be fitted to the wheels of road and rail vehicles, or to the undercarriages of aircraft. The benefits of the brake would accrue in its fitment to any road vehicle, but are especially great in the case of vehicles having high axle loads and operating with long service intervals or under dirty conditions, such vehicles including trucks, coaches, cross-country transport, quarrying and earth-moving equipment, loaders, etc. It is also especially suited for fitment as a front wheel brake for large capacity motor cycles or "superbikes", for reasons explained later. The brake may also be embodied in industrial applications such as transmission brakes, cable or winch brakes, and in any fail-safe application, such as may be used in elevators, ski-lifts, mining gear, etc. Concerning the brake system, this may be conveniently adapted to any vehicle, with the possible exception of large aircraft, but is most suited to incorporation in road commercial vehicles, such as large trucks, and in passenger cars.
The present invention is very much affected indirectly by legislation, either already in force or projected in such developed countries as the U.S.A., Japan, Germany and Scandinavia. The laws relate to the strict imposition of very demanding specifications for vehicle stopping distances and skid control, especially in the case of commercial vehicles. Future heavy trucks will be required to stop in a straight line under all weather conditions in distances being achieved by present passenger cars. Because commercial vehicle axle loads have been progressively increased -- they are now in the region of ten to fourteen tons -- and wheel sizes reduced to accommodate bulkier pay loads, these new stopping distances are barely achievable using existing concepts. The drum and shoe arrangement is at the limit of its development and in these new applications becomes so highly stressed that it becomes unreliable and short-lived, in performance at least. Similar considerations apply to disc brakes where used in trucks, where performance may be slightly better at the cost of even shorter life. Disc brakes have a heat problem (up to seven tons per brake unit may need to be stopped and equivalent energy dissipated, compared with one-fourth to one-half a ton for cars), because in commercial vehicles they cannot be placed in the air stream without unacceptably low life and high maintenance cost. For these reasons it is the conviction of the inventor that only different brake types will in the long term meet commercial vehicle needs. It can be seen that the band brake with its inherent higher performance and consequently lower stressed hydraulic system, its far greater surface area and better load distribution, with consequent better longevity, its lower weight compared to drum and shoe arrangements and, not least, its ability to allow wheel and steering centres to coincide, offers the commercial vehicle manufacturer an answer to braking problems. (The technical considerations mentioned above will be more fully described in later sections.) In practice the band brake would be fitted in association with a modern hydraulic system. For normal car use servo assistance could be eliminated, since the brake is more powerful for a given pedal pressure, improving brake control and "feel". In other applications servo assistance may be introduced either to improve braking effect, or perhaps to reduce it under special conditions, e.g. to prevent low speed lock up.
Before explaining the operational aspects of the invention, it might be appropriate to mention the traditional roles of band brakes. They are today used mainly as transmission, winch and pulley brakes, as well as for fail-safe applications. At one period about the 1920's band brakes were fairly extensively tried as vehicle wheel brakes, but their large scale introduction did not materialise for a number of reasons. These included the fact that they were mostly U-shaped and so transferred hard unequal loads onto the axle during braking, or, if nearly circular, application was by L-shaped projections at the ends of the band, which meant that brake application caused great torsional loads at the ends of the band, resulting in excessive wear at that point and uneven load distribution throughout the band. Band brakes are much sharper and more powerful than drum brakes for equivalent pedal pressures, with the result that braking performance inequalities, often due to the inferior friction materials then used, tended to be magnified. In any case, what were for that period perfectly acceptable stopping distances could be achieved using drum brakes, which were then much larger than today because of the much larger wheel sizes then prevalent, thus, eliminating the motive for extended development of the band brake.
It can be seen from the above comments and the later full description of the invention that the present invention overcomes the major objection to the use of the band brake, which is its previous method of application. With the present brake it is possible to eliminate completely the U configuration and achieve a band brake of almost circular form, and to apply the brake without any torsional forces being set up in the band. Because the tensile forces pass through each other, it is possible to locate the actuating means so far away from the band as to allow for the variation and adjustment of force application points during the progressive tightening of the band, matching the movement to the decreasing circumference and radius of the band. In fact this brake could be called a tensile reducing radius brake, since the band is effectively a circle which shrinks when force is applied. A problem with band brakes on vehicle wheels is that when not applied rapid suspension movement might cause the band to flap or chafe against the revolving drum. This problem is here overcome by the guides used (of which more than two could be employed). To some degree, dependent on whether one or both ends are floating, a band brake will tend to have an on-off servo effect, with the drum tending to pull the band onto it towards one end and push it off at the other. This characteristic feature is no obstacle to the employment of band brakes, which are used very successfully in many applications. This pulling with/pushing against effect, which really describes the forces set up in a circle tending to rotate but which is fixed at one point, can be modified and controlled by design and application of special features of the invention, so that the load distribution can be precisely controlled and/or equally distributed throughout the band. This, which will result in even wear and even radial loads at all points of the circumference, would constitute a significant breakthrough in brake technology and that of band brakes in particular, and is more fully described in the following sections.
Band brakes have significant advantages over other types of brakes, such as the drum and shoe and the disc type in current automotive use. For a given available space and pedal or application force, they produce a many times more powerful braking effect. The band is flexible, allowing for far more even load distribution, and the loads are applied perpendicularly, unlike in drum and shoe arrangements, where at only one point in the rigid arcuate shoe are the loads perpendicular. The very much greater friction material surface area results in longer brake life, as of course does the more even load distribution. Another basic advantage of the band brake is that the interior of the drum is free. This is very important because it means that the drum interior can be occupied by wheel steering pivots such as king pins, allowing wheel centre and steering pivot centre to be as coincidental as the vehicle designer wishes. The handling and safety advantages in having the steering centre as close to wheel centre as possible are well known, and their coincidence is only possible, in the case of wheel mounted brakes, if a band brake is used. If the drum interior does not contain sensitive brake mechanism, then cooling air from the air stream past the vehicle can be ducted through the drum, as indicated in the drawings. The drum may also have interior flanges or fins to act as cooling means, stiffeners or both. The friction material is exposed, so the backs of both the major brake elements may be cooled. Both the two actual braking surfaces are outside the mechanicals of the wheel and communicate with the air stream, so that when the brake is inoperative a controlled air flow may be passed through the gap between braking surfaces.
It has been mentioned that the brake is suitable for various types of vehicles, and also for other applications. It is especially suitable for powerful motorcycles, for the reason that the interior of the drum may be free, i.e. that the drum may consist of a stub cylinder interiorly supported by spokes or flanges, and so admit air transversely through it. At high speed a motorcycle's safety and road adhesion can be seriously affected by side winds which affect particularly the front wheel. Current powerful cycles must employ large disc brakes which do not leave much room for air to pass between the spokes; obviously a band brake can be designed to leave nearly all the wheel unobstructed to lateral air movement, allowing the motorcycle to have improved road adhesion.
The world fuel crises of late 1973, triggered off by the Middle East war of October that year, have caused a complete shift of attitude to engine and vehicle fuel economy to take place in all countries of the world. In the developed countries every possible technical innovation that might lead to fuel saving is being explored, and in the non-oil-producing countries great efforts are being made to reduce oil imports, now up to four times as expensive as fifteen months ago. Here efforts are being directed to restricting vehicle use (for example by imposing speed limits), because fuel saving innovations have not been incorporated on vehicles on the road. So throughout the world there is a great demand for any device which may reduce automotive fuel consumption, either incorporated during manufacture of the vehicle, or fitted afterwards to existing units.
In general vehicle use, by far the greatest proportion of fuel use per distance travelled is spent in accelerating the vehicle, and propelling it up inclines. All the energy expended on these activities is dissipated by braking (the vehicle cannot proceed indefinitely), in the form of conversion of movement through friction into heat which is then lost, or in pumping inoperative fluid through the engine. If any of the energy required to stop or retard the vehicle can be recovered and/or converted into useful work in any manner, then a net fuel saving will have been achieved. The vehicle braking system of the present invention is designed to achieve this, as will be disclosed in later sections, and in some embodiments to convert the energy expended in braking to work propelling the vehicle.
It has been mentioned earlier that much new legislation relating to vehicle stopping distances has been passed, or is contemplated in developed countries. In fact this is a part of a whole spectrum of vehicle safety legislation now being enacted. There is increasing awareness of the unnecessary social, personal and economic disruption and waste caused by automotive accidents, and the legislation is intended to try to reduce accidents to the minimum practical level. There is a demand today, both from the public and sometimes legally from a government, for devices which improve either primary or secondary vehicle safety. The vehicle braking system of the present invention is designed to convert some of the work of, or constructional requirements inherent in, braking to effect improved primary and secondary vehicle safety.